Printing plates are usually clamped on plate holding cylinders by forming an axially extending groove in the plate holding cylinders and locating clamping tongs or clamping jaws in the groove. The clamping jaws can be positively tightened against each other to clamp the plate in the groove, or they can be resiliently spring-pressed against each other. Usually, the leading edge of the plate--with respect to the direction of rotation of the cylinder--is so arranged that the jaws clamp the respective end of the plate fixedly, so that circumferential register can be controlled; the trailing end of the plate, then, is clamped by resilient clamping jaws.
Swedish Publication No. 324,790, Harenza, to which U.S. Pat. No. 3,335,663 and German No. 1,561,007 correspond, describes an arrangement in which, in dependence on the direction of rotation of the cylinder, the clamping jaw holding the leading edge of the plate is fixed in position by a blocking mechanism; the clamping jaw holding the trailing end of the plate is spring-biassed, in order to provide tension on the plate at the trailing end. The arrangement is so made that it is mirror-symmetrical, so that the direction of rotation of the plate cylinder can be freely selected. Depending on the direction of rotation, the jaws are, selectively, either fixedly clamped or spring-loaded against each other, in dependence on which one of the jaws receives the leading and the trailing ends, respectively.
The arrangement as described is complex and additional apparatus must be used in order to permit circumferential register adjustment of the plate. The trailing end of the plate should not be fixedly clamped since plates used in offset and flexo printing frequently have a resilient or soft underlay or are, inherently, furnished with such a material which should be capable of escaping upon application of printing pressure at the printing line, and to prevent formation of a bulge during rolling contact of the plate cylinders against an opposing cylinder.